I. Field of the Invention
The invention relates generally to the field of amplifiers and more particularly to low voltage class AB operational amplifiers.
II. Description of Related Art
Nowadays, electronic accessories are a ubiquitous part of modern day life. Users of portable phones, organizers, and dedicated music and videos players demand electronics offering high performance, convenient size, and sufficient battery life.
A key component of many electronic accessories are the low-power signal processors and audio coder-decoders (CODECS), which are usually battery driven. Low-power, high performance operation is desirable for extending battery life. These goals may be achieved, in part, by using a low voltage supply (less than 2V) for memory, signal processors, and the CODECs used in electronic accessories.
Sometimes CODECs must provide 1 or 2 V RMS (root mean square) outputs to external loads. In this case, the internal amplifiers must operate over a larger supply voltage range, typically 3.6 V. While line outs have a standard impedance of 10 kΩ, the audio amplifier must provide considerably larger currents to headphones which typically have an input impedance of only 32Ω.
Because portable audio and video players are increasingly being used with high fidelity speaker systems, such as in docking stations, automobiles, and home theaters, users have increased expectations for signal to noise ratio (SNR) and total harmonic distortion (THD) performance. For example, headphones may have signal-to-noise ratios of about 70 dB, but on a high-fidelity audio system, amplifier noise (SNR) and distortion (THD) in excess of 100 dB may be noticeable. The player's amplifier, a fundamental aspect of the design, must be able to drive various kinds of loads while dissipating very low power.
Operational-amplifiers (op-amps) have long played a fundamental role in the design of electronic devices of all types. Performance tradeoffs are approached differently by different designs, while occasionally, new technologies and approaches allow performance enhancement in all design considerations.
Class AB op-amps, represent a tradeoff between the inefficient class A design, and the highly distorting class B design. The class A designs have a maximum theoretical operational efficiency of 25%, while the maximum theoretical efficiency of a class B device is 78.5%. Class AB designs generally fall somewhere between the two. Low-voltage op-amp designs require a minimum supply voltage of one gate-sources voltage and two saturation voltages.
A feedback class AB op-amp attempts to monitor and feedback the currents on the output stage of the amplifier. Previous low-voltage designs have failed to accurately monitor the amplifier's output currents with the accuracy needed to achieve SNR and THD performance of 100 dB or better.